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anyone out there helps me please!

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by kiennguyen » Tue Oct 20, 2009 11:41 pm
i got stuck with this question, OA later.

A company plans to assign identification numbers to its employees. Each number is to consist of four different digits from 0 to 9, inclusive, except that the first digit cannot be 0. How many different identification numbers are possible?
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by xcusemeplz2009 » Tue Oct 20, 2009 11:55 pm
IMO 9*7*8*9=4536

OA pls
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by papgust » Wed Oct 21, 2009 1:25 am
Yes, It's 9*9*8*7 = 4536. OA pls?
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by kiennguyen » Wed Oct 21, 2009 2:01 am
your answer is right. can you explain me more detail pls!
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by papgust » Wed Oct 21, 2009 2:36 am
Since we need to select 4 different digits, we must not repeat any of the numbers successively.

Condition: First digit must not be zero.

First digit: We can select 9 numbers for first digit from 10 numbers (0-9) excluding 0 as first digit must not hold 0.

Second: We can select remaining 9 numbers (excluding the no selected for first digit, but NOT excluding 0). Remember we must not repeat numbers!

Third: Out of 10, now we have 8 numbers remaining after selecting first and second digits.

Fourth: Similary, we have 7 numbers remaining out of 10.

Therefore, possibilities for first*second*third*fourth = 9*9*8*7 = 4536.
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